You can never guarantee overclocks. Not every die that comes off the production line has the same amount of headroom - doesn't matter that it's the same model. You could say that 90% of a certain GPU will get a 5% overclock for example, 50% chance that it can gain 15% and 10% chance it can gain 25%. But there's never any guarantees on it. I saw a guy in the forums recently trying to RMA his cards because they weren't overclocking by the as much as other people were getting with the same model.
gtx 660, but overclock it about 5% from stock speed
I'm not bought any GPU cooler
Like others have said, you can't count on overclocking potential. You would need a good overclock out of the 7850 for it to consistently beat the 660. At those prices, I would take option 3 and overclock it 5-10% if you can. You'll be very happy with it.